1. Field of The Invention
The present invention relates to a discharge lamp particularly of low-pressure mercury vapor typically represented by a fluorescent lamp having transparent conductive film coated on the inner surface of the lamp bulb and and also relates to an illuminating apparatus utilizing the same.
2. Description of The Related Arts
A low-pressure mercury vapor discharge lamp typically represented by a fluorescent lamp, such as a rapid-start type fluorescent lamp, has a bulb having an inner surface on which a transparent conductive film having tin oxide is formed as a main component thereof.
The formation of the transparent conductive film of tin oxide is performed, for example, by a spray method in which a tin chloride solution is sprayed or by a so-called CVD method in which vapor of tin oxide is sprayed on the inner surface of the bulb.
Since such a fluorescent lamp offers an excellent starting characteristic and needs no starter lamp, it is widely used in offices, department stores and the like. The rapid-start type fluorescent lamp, however, tends to suffer from yellowish-brown band deposits (hereinafter referred to as "yellowing") and blackening caused on the bulb inner surface in the vicinity of electrodes, for example portions apart from the electrodes by 10 to 30 cm, when it is used for a long time such as about more than 1000 hours. Fluorescent lamps that have no transparent conductive film suffers blackening as well. In this type fluorescent lamp, during the use thereof, electrode materials are sputtered and stuck onto a phosphor film where they react with mercury or phosphor, subsequently leading to blackening. In the rapid-start type fluorescent lamps, however, the yellowing and blackening in the vicinity of the electrodes are attributed to reasons other than the sputtering of electrode materials.
Many papers or publications are known that have analyzed the cause for yellowing and blackening typical of the rapid-start type fluorescent lamps. According to them, why yellowing and blackening take place is roughly summarized as follows. That is, a fluorescent lamp lights on AC supply, microscopic discharges takes place between the transparent conductive film, and electrodes at the moment the polarity is reversed. The microscopic discharges cause the transparent conductive film to change in quality. Mercury stuck onto the phosphor film coating near the electrodes forms a discharge path. Thus, phosphor coating to which mercury sticks is damaged by high energy of the discharge, the phosphor film coating itself reacts with mercury, and furthermore, mercury reacts with the transparent conductive film.
To restrict the generation of the yellowing and blackening phenomenon, microscopic discharge likely to cause them must be controlled. A method known to control the discharge is that the electrical resistance of the transparent conductive film is set higher near the electrodes, as disclosed, for example, in Japanese Patent Application Laid-open No. 56-84861.
More concretely, in the above prior art document of No. 56-84861, there is disclosed a technique for changing a resistance distribution of a transparent conductive film, in which a resistance at a bulb central portion per unit length in the bulb axial direction is made low and a resistance at bulb end portions near the electrodes is made high to obtain substantially V-shaped resistance distribution of the conductive film between the electrodes of the lamp to thereby suppress a minute discharge near the electrodes. Such technique has been applied to commercially sold rapid-start type fluorescent lamps, in which a resistance at the central portion of the bulb is about 2 k.OMEGA. to 50 k.OMEGA. per 10 cm along the axial length thereof and is about 20 k.OMEGA. to 500 k.OMEGA. per 10 cm along the axial length thereof. In such fluorescent lamps, in order to obtain the V-shaped resistance distribution, the film thickness of the conductive film is changed on the inner surface of the bulb. In the disclosed example, the conductive film is formed by introducing vapor of tin compound and reacted therein so as to deposit the tin oxide on the inner surface of the bulb. In this operation, since a reaction speed is changed in an exponential function with respect to a bulb temperature, the thickness of the conductive film is increased by making high the temperature at the central portion of the bulb.
However, in the conventional technology, since the bulb temperature is made high to supply the tin compound vapor by an amount more than required, the reaction of the conductive film and the production speed thereof are made faster, but an un-reacted tin compound remains, making coarse the film condition. Such un-reacted tin compound is gradually reacted during the lighting. It is also found that, due to the reaction after depositing of the film, the film provides a large gap ratio and less elaboration in density, and accordingly, the thickness of the film is made thick even in the same deposited amount and the same resistance. Moreover, since the undecomposed tin compound is reacted and decomposed during the lighting of a fluorescent lamp, as a product, the resistance of the transparent conductive film is gradually lowered during the use of the fluorescent lamp and, hence, angle of the V-shaped resistance distribution is widened, thus liably causing the blackening phenomenon to the lamp. Furthermore, there causes a case where impure gas is produced through the decomposing reaction of the tin compound, damaging the startability of the fluorescent lamp.
In another method such as disclosed in Japanese Patent Application Laid-open No. 57-32561, there is shown a technique for stabilizing the resistance distribution of the transparent conductive film by adding an additive such as antimony to the conductive film. In this technique, however, it is intended to stabilize the resistance distribution only at the production starting time, and accordingly, it is difficult to obtain a proper resistance distribution of V-shape by adding the antimony.
A further method is that an electrically insulating film is coated on the transparent conductive film, as disclosed, for example, in Japanese Patent Application Laid-open No. 50-12885, Japanese Patent Application Laid-open No. 52-49683, Japanese Patent Application Laid-open No. 52-93184, and others.
The prior art quoted above are able to control the generation of the yellowing and blackening to some degree. In the course of the study of this problem, the inventors have found that, when the transparent conductive film is thinned, some difference is caused in the degree of yellowing and blackening generated even with apparently the same design conditions. Specifically, in some lamps, the yellowing and blackening are more noticeable on one side than on the other side of the lamp, while in other lamps yellowing and blackening take place on both the left- and right-hand sides in a balanced manner and are generally less noticeable.